Dementia is a brain disorder that seriously affects a person's ability to carry out normal daily activities. Among older people, Alzheimer's disease (AD) is the most common form of dementia and involves parts of the brain that control thought, memory, and language. Despite intensive research throughout the world, the causes of AD are still unknown and there is no cure. AD most commonly begins after the age of 60 with the risk increasing with age. Younger people can also get AD, but it is much less common. It is estimated that 3 percent of men and women ages 65 to 74 have AD. Almost half of those ages 85 and older may have the disease. AD is not a normal part of aging. Alzheimer's disease is a complex disease that can be caused by genetic and environmental factors. In the United States alone, four million adults suffer from Alzheimer's disease (AD). Not only does Alzheimer's disease significantly impact the lives of countless families today, it threatens to become even more of a problem as the baby boom generation matures. The economic burden of AD in the United States is estimated to cost over $100 billion a year and the average lifetime cost per patient is estimated to be $174,000. Unfortunately, there is no cure available for AD.
In 1906, Dr. Alois Alzheimer, noticed changes in the brain tissue of a woman who had died of an unusual mental illness. In her brain tissue, he found abnormal clumps (now known as amyloid plaques) and tangled bundles of fibers (now known as neurofibrillary tangles) which, today, are considered the pathological hallmarks of AD. Other brain changes in people with AD have been discovered. For example, with AD, there is a loss of nerve cells in areas of the brain that are vital to memory and other mental abilities. Scientists have also found that there are lower levels of chemicals in the brain that carry complex messages back and forth between nerve cells. AD may disrupt normal thinking and memory by blocking these messages between nerve cells.
Plaques and tangles are found in the same brain regions that are affected by neuronal and synaptic loss. Neuronal and synaptic loss is universally recognized as the primary cause in decline of cognitive function. The number of tangles is more highly correlated with the cognitive decline than amyloid load in patients with AD (Albert Proc. Natl. Acad. Sci. U.S.A. 93:13547-13551 (1996)). The cellular, biochemical, and molecular events responsible for neuronal and synaptic loss in AD are not known. A number of studies have demonstrated that amyloid can be directly toxic to neurons (Iversen et al. Biochem. J. 311:1-16 (1995); Weiss et al. J. Neurochem. 62:372-375 (1994); Lorenzo et al. Ann. N.Y. Acad. Sci. 777:89-95 (1996); Storey et al. Neuropathol. Appl. Neurobiol. 2:81-97 (1999), resulting in behavioral impairment. The toxicity of amyloid or tangles is potentially aggravated by activation of the complement cascade (Rogers et al. Proc. Natl. Acad. Sci. U.S.A. 21:10016-10020 (1992); Rozemuller et al. Res. Immunol. 6:646-9 (1992); Rogers et al. Res. Immunol. 6:624-30 (1992); Webster et al. J. Neurochem. 69(1):388-98 (1997)). This suggests involvement of an inflammatory process in AD and neuronal death seen in AD (Fagarasan et al. Brain Res. 723(1-2):231-4. (1996); Kalaria et al. Neurodegeneration 5(4):497-503 (1996); Kalaria et al. Neurobiol Aging. 17(5):687-93 (1996); Farlow Am. J. Health Syst. Pharm. 55 Suppl. 2:S5-10 (1998).
Evidence that amyloid β protein (Aβ) deposition causes some forms of AD was provided by genetic and molecular studies of some familial forms of AD (FAD). (See, e.g., Ii Drugs Aging 7(2):97-109 (1995); Hardy Proc. Natl. Acad. Sci. U.S.A. 94(6):2095-7 (1997); Selkoe J. Biol. Chem. 271(31):18295-8 (1996)). The amyloid plaque buildup in AD patients suggests that abnormal processing of Aβ may be a cause of AD. Aβ is a peptide of 39 to 42 amino acids and forms the core of senile plaques observed in all Alzheimer cases. If abnormal processing is the primary cause of AD, then familial Alzheimer's disease (FAD) mutations that are linked (genetically) to FAD may induce changes that, in one way or another, foster Aβ deposition. There are 3 FAD genes known so far (Hardy et al. Science 282:1075-9 (1998); Ray et al. (1998)). Mutations in these FAD genes can result in increased Aβ deposition.
The first of the 3 FAD genes codes for the Aβ precursor, amyloid precursor protein (APP) (Selkoe J. Biol. Chem. 271(31):18295-8 (1996)). Mutations in the APP gene are very rare, but all of them cause AD with 100% penetrance and result in elevated production of either total Aβ or Aβ42, both in model transfected cells and transgenic animals. The other two FAD genes code for presenilin 1 and 2 (PS1, PS2) (Hardy Proc. Natl. Acad. Sci. U.S.A. 94(6):2095-7 (1997)). The presenilins contain 8 transmembrane domains and several lines of evidence suggest that they are involved in intracellular protein trafficking. Other studies suggest that the presenilins function as proteases. Mutations in the presenilin genes are more common than in the APP gene, and all of them also cause FAD with 100% penetrance. Similar to APP mutants, studies have demonstrated that PS1 and PS2 mutations shift APP metabolism, resulting in elevated Aβ42 production (in vitro and in vivo).
Cyclooxygenases (COX) are major Alzheimer's disease drug targets due to the epidemiological association of NSAID use, whose primary target are cycloxygenases, with a reduced risk of developing Alzheimer's disease (see, e.g., Hoozemans et al. Curr. Drug Targets 4(6):461-8 (2003) and Pasinetti et al. J. Neurosci. Res. 54(1):1-6 (1998)). The epidemiological studies have indicated that chronic NSAID use appears to reduce the risk of acquiring Alzheimer's disease and/or delay the onset of the disease (see e.g., McGeer et al. Neurology 47(2):425-432 (1996); and Etminan et al. BMJ. 327(7407):128 (2003)). COX-2 selective inhibitors are attractive candidates for long-term drug use since they do not inhibit COX-1 and appear to be less toxic. In support of COX-2 as a target for the treatment for AD, a recent study was published reporting that in mouse models of AD, COX-2 overexpression was related to the neuropathology of AD (Xiang et al. Neurobiol. Aging 23:327-34 (2002)). However, recent clinical trials of specific NSAIDs have called into question the hypothesis the hypothesis that anti-inflammatory drugs are useful for the treatment or prevention of Alzheimer's disease. It was reported that rofecoxib, a COX-2 selective NSAID, at 25 mg daily, failed to show efficacy for treating AD. Naproxen, another NSAID, in the same trial failed to show efficacy in Alzheimer's treatment. See Aisen et al. JAMA 289:2819-26 (2003) and Reines et al. Neurology 62(1):66-71 (2004). These authors concluded that the results with naproxen and rofecoxib do not support the use of NSAIDs for the treatment of AD. Celecoxib, a COX-2-selective NSAID, failed to show efficacy in several recent clinical trials for the treatment of AD. See Jhee et al., “A Double-Blind, Placebo-Controlled Pharmacokinetic (PK), Pharmacodynamic (PD) and Safety Study of Celecoxib Treatment for Four Weeks in Patients with Alzheimer's Disease (AD),” Abstract from 7th International Geneva/Springfield Symposium on Advances in Alzheimer's Therapy (2002); also published in Clinical Research and Regulatory Affairs 21(1): 49-66 (2004)) and Sainati et al. (Abstract from 6th International Stockholm/Springfield Symposium on Advances on Alzheimer's Therapy, Abstract Book 2000; 180). Conversely, it was reported recently that rofecoxib provides neuroprotection in an in vivo Alzheimer's disease excitotoxic model system (Scali et al. Neuroscience 117:909-919 (2003). However, rofecoxib, in a large prevention clinical trial, failed to prevent the development of Alzheimer's disease in patients having mild cognitive impairment. In fact, the results of this trial showed that 6.4% of patients taking rofecoxib developed AD as compared to 4.5% for those taking placebo (see e.g., Visser et al., abstract from Annual meeting of the American College of Neuropsychopharmacology San Juan, Puerto Rico, 2003; and Landers, Wall Street Journal 10 Dec. 2003). Thus, clinical trials have indicated that NSAIDs, as a general class of drugs, are not likely to be useful for treating and/or preventing Alzheimer's disease.
Of the five drugs currently being used in the US for the treatment of AD, four of them—tacrine (Cognex®), donepezil (Aricept®), rivastigmine (Exelon®), and galantamine)(Reminyl®)—are inhibitors of acetylcholinesterase. Another drug, memantine, was recently approved for treating moderate-to-severe AD. More recently it was reported that memantine showed efficacy in treating mild-to-moderate AD. Memantine is a NMDA receptor antagonist.
The drugs currently used for treating AD, including memantine and the acetylcholine esterase inhibitors, are marginally efficacious and have undesirable side-effects. Thus, there is a large unmet need for better and safer drugs.